A Ni-based single crystal superalloy is used as a material for components or products which are used for long periods of time under high temperature, such as for a blade or a vane used in jet engines for airplanes or gas turbines. The Ni-based single crystal superalloy is a superalloy obtained by adding Ni (nickel) as a base to Al (aluminum) so as to give a Ni3Al type precipitate for strengthening, then mixing with metal having high melting point such as Cr (chrome), W (tungsten) and Ta (tantalum) to give an alloy, and making it into a single crystal. As the Ni-based single crystal superalloy, a first generation superalloy not including Re (rhenium), a second generation superalloy including about 3 wt % of Re, and a third generation superalloy including 5 to 6 wt % of Re have already developed, and creep strength has been improved with the advance of generation. For example, CMSX-2 (produced by Canon-Muskegon Corporation, see Patent Document 1) has been known as a first generation Ni-based single crystal superalloy, CMSX-4 (produced by Canon-Muskegon Corporation, see Patent Document 2) has been known as a second generation Ni-based single crystal superalloy, and CMSX-10 (produced by Canon-Muskegon Corporation, see Patent Document 3) has been known as a third generation Ni-based single crystal superalloy.
The Ni-based single crystal superalloy is subjected to a solution treatment at a predetermined temperature and then subjected to an aging treatment to obtain a metal constitution with improved strength. This superalloy is referred to as a so-called precipitation hardening-type alloy, which has a constitution including a matrix (γ phase) as an austenite phase and a precipitated phase (γ′ phase) dispersed and precipitated in the matrix as an intermediate regular phase.
The CMSX-10 which is a third generation Ni-based single crystal superalloy is produced for the purpose of achieving improved creep strength under high temperature compared with a second generation Ni-based single crystal superalloy. However, since the content of Re is high, specifically, 5 wt % or more, and exceeds the amount of a solid solution of Re in the matrix (γ phase), the remaining Re combines with other elements and a so-called TCP (Topologically Close Packed) phase is precipitated under high temperature. As a result, the amount of the TCP phase increases due to the long-time use under high temperature and thus a problem occurs in that the creep strength lowers.
In order to solve the problem of the third generation Ni-based single crystal superalloy, Ru (ruthenium) suppressing the TCP phase has been added and contents of other constituent elements have been set to their optimum ranges to adjust a lattice constant of the matrix (γ phase) and a lattice constant of the precipitated phase (γ′ phase) to their optimum values and thus a Ni-based single crystal superalloy with improved strength under high temperature has been developed. Such a Ni-based single crystal superalloy includes a fourth generation superalloy including up to about 3 wt % of Ru and a fifth generation superalloy including 4 wt % or more of Ru, and the creep strength improves in accordance with the advancement of generations. For example, TMS-138 (produced by NIMS-IHI, see Patent Document 4) has been known as a fourth generation Ni-based single crystal superalloy and TMS-162 (produced by NIMS-IHI, see Patent Document 5) has been known as a fifth generation Ni-based single crystal superalloy.
The TMS-138 as a fourth generation Ni-based single crystal superalloy and the TMS-162 as a fifth generation Ni-based single crystal superalloy are superalloys which have improved creep strength, as described above. However, when test pieces are heated at 1100° C. for 500 hours, it is found that the weight change is greater in the negative direction.
When an elemental map of a cross-section of a blade made of TMS-138 after a jet engine test was analyzed, oxides of Ni and Co (cobalt) were distributed in the form of a layer, and under the oxides, an oxide of Al or Cr was distributed in the form of grains on the outermost surface of the blade. When the oxide of Al is formed in the form of a layer, the growth is slow and stable, and it becomes solid, and thus it acts as an oxidation resistant protective film. However, the oxides of Ni and Co grow fast and their adhesion with a base material is lower than the oxide of Al and thus peeling occurs. Accordingly, the peeling phenomenon occurs as the oxidation proceeds, and the weight change in the negative direction increases. That is, a large weight change indicates that the oxidation resistance is not excellent.
[Patent Document 1] U.S. Pat. No. 4,582,548
[Patent Document 2] U.S. Pat. No. 4,643,782
[Patent Document 3] U.S. Pat. No. 5,366,695
[Patent Document 4] U.S. Pat. No. 6,966,956
[Patent Document 5] US Patent Application, Publication No. 2006/0011271